(1) Field of the Invention
The present invention relates to a method for controlling combustion of burners in a batch-type combustion furnace, which method enables the combustion of each of the burners to be controlled, while always maintaining the combustion state of the burners at not less than a theoretical air-fuel ratio xcex=1. [xcex=(actual air volume)÷(air volume for the stoichiometric combustion of a given fuel gas volume)].
(2) Related Art Statement
In order to control the combustion of the burners in the batch-type combustion furnace, there are known two methods: (i) is a method using a pressure-equalizing valve and (ii) is a method of controlling a flow ratio between a fuel gas and air through independently measuring flow rates of the fuel gas and the air, respectively. Between them, according to the first method of using the pressure-equalizing valve method, as shown in FIG. 5, the pressure Pg of the fuel gas is always made equal to that of the Pa of the air in an air pipeline by a pressure-equaling valve 1 which is arranged in a fuel gas pipeline, and the respective flow rates of the fuel gas and the air are adjusted by throttle valves 2 and 3, respectively, so that the flow rate of the fuel gas may be in proportion to that of the air. Combustion is controlled by opening or closing an air control damper 5 with a temperature controller 4. When the air-fuel ratio is to be changed, a part of a loading pressure upon the pressure-equalizing valve 1 is released through a loading pressure release valve 6 thereby to increase the fuel-air ratio. In FIG. 5, a thermocouple 7 is connected to the temperature controller 4, and a combustion air blower 8 to the air control damper. A loading pressure program 9 is connected to the loading pressure release valve 6. A burner is denoted by 10.
Since in this method the controlling system can be relatively inexpensively constructed, it is suitable for independently controlling respective burners. However, since a variable range of the fuel gas pressure Pg is as small as 2 to 20 kPa due to the performance of the pressure equalizing-valve 1, the maximum air-fuel ratio xe2x80x9cxcexxe2x80x9d is as small as around 4.0. Owing to this limitation, the above method is not suitable for the batch-type combustion furnace requiring the air-fuel ratio xe2x80x9cxcexxe2x80x9d in a low temperature range to be controlled to 10 or more.
The present invention has been made to provide a method for controlling the combustion of burners in a batch-type combustion furnace where the method solves the conventional problems mentioned above, realizes controlling the air-fuel ratio xe2x80x9cxcexxe2x80x9d to a high level of 10 or above, and is suitable when viewed from performance and price.
The present invention, which has been made to solve the above-mentioned problems, relates to a method for controlling each of the burners in a batch-type combustion furnace in which a fuel gas control valve is electrically linked to a corresponding air control valve. The method comprising the steps of preliminarily affecting a flow range ratio adjusted with zero and span adjustments between the fuel gas control valve and the corresponding air control valve so as to give a theoretical air-fuel ratio when equal electric signals are given to the fuel gas control valve and the air control valve. The next step is inputting a first electric signal from a temperature controller to the fuel gas control valve to control an open degree of the fuel gas control valve, then adding an electric signal for adjusting an excess air volume to a second electrical signal equal to the first one for the fuel gas control valve to control an open degree of the air control valve, and finally inputting the resulting electric signal obtained by the above addition to the air control valve, wherein the combustion of each of the burners is controlled.
The excess air volume-adjusting electrical signal is preferably determined based on the content of oxygen inside the furnace and an excess volume of oxygen required in the furnace, which excess volume is a set valve at a given point of time of a time vs. firing temperature program. Further, it is preferable to control, at a constant level, the air pressure of an air source to supply combustion air to the air control valve. Further, it is preferable that if a trouble signal is outputted from the fuel gas control valve or the air control valve, the combustion of the burner is interrupted.
Since the present invention adopts the system in which the fuel gas control valve and the air control valve are electrically linked to control the combustion of each of the burners, the control system can be constructed inexpensively. Further, the zero span adjustment is preliminarily effected between the fuel gas control valve and the corresponding air control valve in such a manner as to give a theoretical air-fuel ratio when equal electric signals are given to the fuel gas control valve and the air control valve, respectively. Then in operation the electric signal obtained by adding the electric signal for adjusting an excess air volume to the electrical signal equal to that for the fuel gas control valve is inputted to the air control valve. Accordingly, the air-fuel ratio can be electrically and arbitrarily controlled.